FCC closed cyclone with snorkel

ABSTRACT

Thermal cracking in a riser cracking, closed cyclone, fluidized catalytic cracking process is reduced. A snorkel or flow conduit having an inlet just above the catalyst stripper moves stripper vapor into the closed cyclone. The system preferably operates without a stripper cap, relying on fluid dynamics to isolate stripper vapor from upper parts of the vessel containing the riser outlet. Preferably the snorkel is at least partially supported by, and ideally is inside, the primary cyclone dipleg. Reduced residence time of stripper vapor in the vessel containing the stripper and the closed cyclone system reduces thermal cracking of stripper vapor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of my prior co-pending application Ser.No. 10/621,197 filed Jul. 16, 2003, which is incorporated by reference,and claims the benefit of my prior provisional application 60/397,797filed on Jul. 23, 2002

FIELD OF THE INVENTION

This invention relates to fluid catalytic cracking and more particularlyto a closed cyclone system for separating cracked hydrocarbons fromspent cracking catalyst discharged from a riser reactor.

BACKGROUND OF THE INVENTION

The fluid catalytic cracking (FCC) process has become well-establishedin the petroleum refining industry for converting higher boilingpetroleum fractions into lower boiling products, especially gasoline.

In the FCC process, a finely divided solid cracking catalyst promotescracking reactions. The catalyst is in a finely divided form, typicalparticle size of 20-100 microns with an average of about 60-75 microns.The catalyst acts like a fluid (hence the designation FCC) andcirculates in a closed cycle between a cracking zone and a separateregeneration zone.

In the cracking zone, hot catalyst contacts the feed so as to effect thedesired cracking reactions and coke up the catalyst. The catalyst isthen separated from cracked products which are removed from the crackingreactor for further processing. The coked catalyst is stripped and thenregenerated. A good overview of the importance of the FCC process, andits continuous improvement, is reported in Fluid Catalytic CrackingReport, Amos A. Avidan, Michael Edwards and Hartley Owen, in the Jan. 8,1990 Oil & Gas Journal.

One of the most significant problems remaining is post-riser cracking.As FCC technology has improved, post riser cracking has gone from atrivial problem which was hard to find in a commercial unit to a majorproblem which can not be ignored.

The problem could be largely ignored when the FCC unit operated with ariser top temperature of 950° F. As catalysts became more active, andfeeds heavier, riser top temperatures increased above 1,000° F. in manyunits. The large volume “reactor”—in which the riser reactordischarged—became a thermal cracker. Large amounts of cracked productvapor spent a significant amount of time in the vapor space above theFCC stripper, and time and high temperature predictably produced thermalcracking.

More details about the problem and the state of the art methods ofminimizing such thermal cracking, are presented in FCC CLOSED CYCLONESYSTEM ELIMINATES POST-RISER CRACKING, by Amos A. Avidan, Frederick J.Krambec, Hartley Owen, and Paul H. Schipper, presented at the 1990 NPRAAnnual Meeting, Mar. 25-27, 1990.

The authors, one of whom is the present inventor, reported the dramaticdecrease in thermal cracking which could be achieved using a“closed-cyclone” system to separate cracked products from spent catalystas they are discharged from the FCC riser reactor. The paper reviewedvarious “rough cut” separation devices (in U.S. Pat. No. 4,295,961 andU.S. Pat. No. 4,664,888 and U.S. Pat. No. 4,721,603, which areincorporated by reference). While they are improvements over dense bedcracking units or riser cracking units with no cyclones, the rough cutdevices merely separated spent catalyst from spent products. The roughcut cyclones still allowed a significant amount of hydrocarbons toremain a long time in the reactor vessel, the volume above the stripper.

The authors presented a new type of riser reactor cyclone, a “closedcyclone” which effectively separated cracked products from spentcatalyst and quickly removed the cracked products from the reactorvessel. Closed cyclone designs are reported in U.S. Pat. No. 5,055,177,Haddad et al. This design, and the closed cyclone designs of other oilcompanies, generally did an excellent job of quickly removing from thevessel the cracked vapors recovered via the riser cyclones. This designignored another problem, thermal cracking of stripper vapor.

A significant amount of the cracked vapor product stays with, or isentrained in, or is needed to fluidize, the spent catalyst. Thoseworking in this field concentrated their efforts on the primary productvapor stream, the 90+ mole % of the cracked vapor recovered as a vaporphase out of the reactor vessel. They generally ignored the secondaryvapor product, the modest amount of vapor product discharged with thespent catalyst from the reactor cyclone systems. This vapor product,plus additional hydrocarbons displaced or desorbed from the spentcatalyst by steam stripping, was at a temperature approaching that ofthe riser reactor. Although small in size, the secondary product couldcrack thermally, and was severely over-cracked even as the primaryproduct was removed relatively unscathed due to thermal cracking.

Thermal cracking depends on time and temperature. The net effect ofgetting most of the vapor out quickly (to practically eliminate thermalcracking of this material) was significantly offset by large amounts ofthermal cracking of the secondary vapor product in the reactor volumeabove the stripper.

An order of magnitude less hydrocarbon was discharged down the risercyclone diplegs to be recovered with stripper vapors. The reactor volumestayed the same, so as closed cyclone efficiency increased, the strippervapor residence time increased dramatically.

Refiners reported dramatic yield benefits. The Avidan et al paperpresented at the 1990 NPRA meeting reported a reduction of 40 percent insulfur-free dry gas make. This was a significant and noteworthyaccomplishment, but was actually a combination of two phenomena—an evenmore dramatic reduction in dry gas make in the primary product and anoffsetting significant increase in thermal cracking and attendant drygas production in the secondary, or stripper vapor product.

Another benefit of closed cyclones reported in this paper was that theproduction of butadiene is reduced by over 50 percent. Butadiene is asensitive measure of thermal cracking in FCC, refiners watch it becauseit is a major contributor to acid consumption in downstream alkylationunits.

I wanted to retain the benefits of closed cyclone FCC riser cracking,but avoid the excessive thermal cracking which occurred in the reactorvolume. My goal could also be considered as a way to help solve a longstanding problem, post riser thermal cracking in the vessel volume abovethe FCC stripper.

Four approaches have been proposed or used to reduce thermal cracking inthis area: dome steam, riser quench, post-riser steam quench and astripper cap/snorkel. To me all these are related and are attempts toreduce thermal cracking in a catalytic cracking unit. Each approach hasbenefits and burdens, and each is reviewed below.

Dome Steam

Refiners have known for years that thermal cracking went on in the“reactor volume” above the stripper. Thus in addition to the four kindsof coke make associated with FCC (catalytic, CCR, Pt function, andcat/oil), refiners have known about “dome coke”—a product of undesiredthermal cracking in the dome of the reactor vessel, or the vesselholding the stripper. Unless this part of the vessel is continuouslypurged with steam, the relatively stagnant region allows thermalcracking to proceed unabated, which produces large coke deposits whichcan grow in size, break off, and damage vessel internals. Refiners nowadd 500 to 1000 #/hour of steam directly to the dome volume. This purgesteam sweeps hydrocarbons out of the dome region. Refiners havepracticed this for decades, but it is so well accepted and universallypracticed that it is rarely discussed. I mention it to show thatrefiners are well aware of the problem of thermal cracking in the vesselvolume.

While use of dome steam has been practiced for decades, another attemptat suppressing thermal cracking was commercialized in recent years—riserquench.

Riser Quench

Refiners have known that higher riser temperatures are beneficial incracking heavy feeds in a short time. They have also known that highertemperatures in the riser lead to undesired thermal cracking downstreamof the riser in the reactor vessel. A way to have higher temperatures inthe riser than in the reactor vessel is to use riser quench. Severalapproaches to quench have been developed, as reported in U.S. Pat. No.4,818,372, Mauleon et al and U.S. Pat. No. 5,389,232, Adewuyi et al,which are incorporated by reference. Basically riser quench involvescracking the feed in the base of the riser at a higher than normaltemperature and injecting a cooler material, such as light cycle oil,higher up in the riser.

Some refiners quench quickly, within less than a second of residencetime in the riser, while some quench higher up or even at the riseroutlet. This will reduce post-riser thermal cracking. A somewhat relatedapproach is post-riser quenching, discussed next.

Post-Riser Quenching

U.S. Pat. No. 4,978,440, Krambeck, Dec. 18, 1990, taught injecting wateror steam downstream of the FCC riser. This patent is incorporated byreference.

The patentee recognized that thermal cracking occurred in the reactorvessel. Closed cyclone operation increased the residence time of thestripper vapor. The solution, adding steam to reduce the temperature inthe vapor phase above the stripper, reduced thermal cracking. It alsorequired adding a large amount of steam to the FCC unit, and this steamtied up a significant portion of the plant volume with water vapor.

Stripper Cap

A fourth approach was isolation of the stripper, a stripper cap andsnorkel, as disclosed in U.S. Pat. No. 4,946,656, VENTED STRIPPERSECTION FOR A FLUID CATALYTIC CRACKING UNIT APPARATUS, which isincorporated by reference. They proposed to isolate the stripper fromthe interior volume of the vessel 1 with a stripper cap 40. This vesselcontained the outlet of the riser reactor, the closed cyclone separationsystem, and the catalyst stripper. The stripper cap was a slant tray,the slope ensured that catalyst falling on the cap would eventually falldown into the stripper. The isolated stripper vapors were removed fromthe under the stripper cap via a “chimney vent” line 30 passing throughholes 29 in the stripper cap. The vent line tied in with the vapor linefrom the primary cyclone 5 to the secondary cyclone 9.

The patentees recognized the problem—thermal cracking of stripper vaporin closed cyclone FCC operation. Their solution, if implemented, wouldpartially solve the thermal cracking problem while creating otherproblems.

Their solution would efficiently remove much of the stripper vapor andreduce—but not eliminate—thermal cracking of stripper vapor in thevessel 1. Thus it was at least a partial solution to the problem, withthe part left unsolved being the undesired thermal cracking of strippervapor which would pass through the holes in the cap to the interior ofvessel to eventually leave via an annulus in the upper part of vessel 1.

Implementing this solution in commercial FCC units would cause someproblems. First the construction and servicing of the unit are greatlycomplicated by the addition of stripper cap 40. It physically isolatesthe stripper from everything else, making it harder to inspect, work on,or repair internal stripper hardware. Second, the cap has to bemechanically strong. If any part of it falls off the operation of thecatalyst stripper will suffer greatly. The cap has to be segmented, sothat it can be fit through man-ways providing access to vessel 1. Thecap will generally be installed around the cyclone diplegs and the riserreactor, so a complex field fabrication procedure will be required.

The support of chimney vent 30 causes significant problems, at least inthe embodiment shown in the patent. The vapor inlet to the secondarycyclone, line 23, is supported entirely by the secondary cyclone. Thisline in turn supports stripper vent 30, while a portion of line 23 mustfit loosely over the primary cyclone vapor outlet, line 21. A loose fitis necessary because the stripper cap has so many holes in it that asignificant amount of stripper vapor will make its way through thereactor vessel volume and pass through annulus 27.

The mechanical design of such a system is complex and costly. The systemmust accommodate a significant amount of thermal expansion. Just as theSR71 Blackbird is reported to grow in length almost a foot, as it heatsup during supersonic flight, an FCC riser gets longer as it heats up.The riser heats up first, followed by the cyclones, so they do notexpand simultaneously. In addition to thermal stress, the FCC riser canbump and even shake at times if something goes off on cat/oil ratio orsteam addition in some part of the unit.

An additional concern with the vent cap design shown is possibleformation of dome coke, or perhaps “vent cap coke”, in stagnant portionsof the cap. Even though steam is added in large amounts to the stripper,and apparently to the stripper volume via line 44, the apex regions ofthe vent cap will be relatively stagnant and difficult to purge withsteam. Coke formation is a distinct possibility. Once coke formationstarts, it will continue. It is possible to form large pieces of cokewhich can fall off and impair stripper operation and may even present asafety hazard during turnarounds. It may be possible to design a ventcap purge which would be as effective as dome steam but this is not atrivial problem, as this area is so difficult to reach. If a small steamline is put in it may not be there in a few years (due to erosion),while if a large line is put in there are problems supporting it andconnecting it to a “moving target”, which will be the case due tothermal expansion.

The most troublesome concern is that the design seems to allow asignificant amount of hydrocarbon vapor traffic from the stripper upthrough the reactor vessel into annulus 27. A significant amount ofthermal cracking of this material as it passes through the large voidvolume of the reactor vessel seems likely. The patentees recognizedthis, and proposed adding more steam via steam spargers 34 and 36 tospeed this material on its way. While this addition will mitigate thedamage done to stripper vapor during its passage through the reactorvessel, it seemed more like treating the symptom rather than thedisease.

I know how commercial FCC units operate. It is my belief that noneachieve the full potential of riser cracking plus closed cycloneoperation. Most refiners have been happy to have the benefits of hightemperature riser cracking and look on the thermal cracking as a burdenof higher riser temperatures.

While quenching, either in the riser or downstream, reduces thermalcracking, significant amounts of quench material are needed. Closedcyclone operation provided the most benefit with the least burden. Thebenefits of closed cyclones were significant, but the full potential wasusually not seen because some of the improvement was masked by thesignificant increase in thermal cracking of stripper vapor.

I wanted a better way to reduce thermal cracking of stripper vapor in anFCC unit using a closed cyclone system on the riser outlet. I acceptedthat a stripper was essential for this type of FCC operation, but didnot, however, believe this material had to stagnate and crack in thereactor vessel above the stripper. Vapor had to be with the catalystflowing down the primary cyclone diplegs into the stripper, but thisvapor (as well as stripping steam) did not have to pass through thevessel volume above the stripper. All the stripper vapor did in thisvolume was thermally crack, and I wanted to get it out, but in a waywhich was compatible with the unit as it stood. I wanted a complete,rather than a partial solution to the problem of thermal cracking ofstripper vapor in the reactor vessel.

Refiners wanting to reduce thermal cracking in the reactor void volumehad few good options. It was possible to quench, either the entirecontents of the riser reactor, or just the stripper vapor, but this haddrawbacks. It would be possible to reduce thermal cracking by reducingthe reactor volume, but this would involve an exorbitant capitalexpense. It was possible to use a stripper cap but a difficult andtroublesome installation was involved. The stripper cap was a good,approach, but only a partial solution. The cap had holes in it whichpermitted passage of some stripper vapor through the reactor vesselvolume.

I wanted to remove stripper vapor promptly to reduce thermal cracking,but did not want to have to physically isolate the stripper from reactorvessel holding the riser cyclones and riser outlet. I realized it waspossible to rapidly and effectively remove stripper vapor from above thestripper using fluid dynamics rather than a cap full of holes to isolatethe stripper. The simplest implementation was leave the cap off andsimply use a snorkel or vapor flow tube extending from slightly abovethe stripper to some portion of the closed cyclone system.

Preferably, the cyclones are entirely closed, with no opening into thecyclone system except for the riser outlet and my snorkel. Closing theannular cyclone openings used in the prior art turns flow in the reactorvessel upside down. Instead of hydrocarbons rising in the reactor vesseland being thermally cracked, there is vapor phase down-flow in thereactor vessel. The dome steam, preferably augmented by modest amountsof additional steam, continually forces vapor to flow down through thereactor vessel. Vapor from the riser has only two ways out of theunit—the vast majority leaves rapidly via cyclone vapor outlets while aminority (stripped hydrocarbons and stripping steam) leaves via thesnorkel sealingly connected to the vapor line from the primary to thesecondary cyclones.

My snorkel's function was somewhat analogous to a snorkel for asubmerged submarine, with the snorkel extending just above the surfaceof the water, supplying air to the diesel engines. My cyclone snorkelextends down, rather than up, but its inlet should terminate just abovethe surface, the top level of fluidized catalyst in the FCC catalyststripper.

I even discovered a way to support the snorkel—simply affix it to thedipleg of one or more of the primary cyclones. My snorkel is small, andhandles a relatively small stream, a small amount of vapor (relative tothe vapor stream discharged from the riser) with a modest amount ofentrained catalyst.

Using the cyclone dipleg to support the snorkel allows the snorkel to beplaced where it is needed, and supported without significantly adding“torque”. There will be little or nor problem due to differentialthermal expansion, the temperature of the vapor passing up the snorkelwill always be about the same as the temperature of the spent catalystphase being discharged down the primary cyclone dipleg. Ideally, thesnorkel is axially aligned with the cyclone dipleg, and discharges upinto the primary cyclone vapor outlet. This will add a modest amount ofsolids (entrained with the stripper vapor) to the vapor phase dischargedfrom the primary cyclone, but the secondary cyclone is equipped tohandle such modest amounts of solids. The snorkel may also be within theprimary cyclone dipleg, attached to a sidewall thereof, or even attachedto the outside of the primary cyclone dipleg for much or all of itsvertical travel.

The closed cyclone snorkel provides a way for most of the stripper vaporto be removed quickly from the reactor, without passing through thelarge void volume of the vessel containing the riser outlet and thestripper inlet. There will still be some hydrocarbon in the void volume.The void volume will be adequately purged using the existing dome steaminjection required to prevent dome coke. If desired, thermal crackingmay be even further reduced by increasing the amount of dome steam, toquench to some extent the hydrocarbons present in the reactor voidvolume, or additional steam may be added at different elevations. Iprefer to minimize this type of steam addition, and my process will workwith 500 to 5000 lbs/hr of “dome steam.”

In addition to discovering a simpler way to isolate stripper vapor fromthe reactor vessel, I discovered a mechanically superior type of snorkelarrangement which facilitates installation of the device.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the present invention provides a fluidized catalyticcracking process for cracking a crackable hydrocarbon feed by contactwith a source of regenerated fluidized cracking catalyst in an enclosedconduit cracking reactor to produce a mixture of cracked hydrocarbonproducts and spent catalyst containing coke and strippable hydrocarbonsand discharging said mixture directly into a closed cyclonic separationmeans within a vessel; then cyclonically separating said mixture in saidseparation means into a cracked hydrocarbon product vapor phase with areduced catalyst content relative to said mixture discharged from saidcracking reactor and a spent catalyst phase containing coke andstrippable hydrocarbons; discharging said spent catalyst phase down fromsaid cyclonic separation means into a catalyst stripper within saidvessel, said stripper having an upper portion with a stripper crosssectional area, said stripper being in open fluid communication withsaid vessel and at least a majority of said stripper cross sectionalarea is open to said vessel; stripping said spent catalyst in saidcatalyst stripping means by maintaining spent catalyst as a dense phasefluidized bed fluidized at least in part by injection of stripping steaminto a lower portion of said bed to produce stripper vapor which isdischarged up from said dense phase fluidized bed in said strippingmeans into said vessel, and stripped catalyst which is discharged fromsaid stripping means into a catalyst regenerator; regenerating saidstripped catalyst in said catalyst regenerator at catalyst regenerationconditions including contact with an oxygen containing gas and burningcoke from said stripped catalyst to produce regenerated catalyst whichis recycled to said cracking reactor to crack said feed; and recoveringsaid stripper vapor discharged up from said dense phase fluidized bed insaid stripping means via a snorkel comprising a vertically extendingtransfer conduit having an inlet in said upper portion of said stripperabove said dense phase fluidized bed of spent catalyst in said stripperand an outlet connective with said cyclonic separation means.

In another embodiment, the present invention provides a fluid catalyticcracking process comprising cracking a crackable hydrocarbon feed bycontact with a source of regenerated fluidized cracking catalyst in anenclosed conduit cracking reactor to produce a mixture of crackedhydrocarbon products and spent catalyst containing coke and strippablehydrocarbons and having a temperature above 1000° F. and sufficientlyhigh to cause thermal cracking of cracked hydrocarbon products anddischarging said mixture from said enclosed conduit directly into aclosed cyclone separator system comprising primary and secondary cycloneseparators within a vessel; cyclonically separating said mixture in saidprimary cyclone separator into: a cracked product vapor phase comprisingat least 90 mole % of said hydrocarbon product vapor discharged fromsaid riser and less than 5 wt % of said spent catalyst discharged fromsaid riser, which is discharged via a primary cyclone vapor outletconnective with an inlet to said secondary cyclone, and a spent catalystphase comprising at least 95 wt % of said spent catalyst discharged fromsaid riser and less than 10 mole % of said vapor discharged from saidriser, which is discharged down via a primary cyclone dipleg into acatalyst stripper in a lower portion of said vessel; cyclonicallyseparating said vapor phase discharged from said primary separator insaid secondary cyclone separator into: a cracked hydrocarbon productvapor phase having less than 1 wt % of said spent catalyst dischargedfrom said riser, which is discharged via a secondary cyclone vaporoutlet to a line connective with a product fractionator, and a spentcatalyst phase, comprising less than 5 wt % of spent catalyst dischargedfrom said riser and less than 2 mole % of vapor discharged from saidriser, which is discharged from a secondary cyclone dipleg into saidcatalyst stripper; stripping in said catalyst stripper spent catalystdischarged from said primary and secondary cyclone diplegs in a densephase fluidized bed fluidized at least in part by injection of strippingsteam to a lower portion of said bed to produce: stripper vapor which isdischarged up from said fluidized bed catalyst stripper, and strippedcatalyst which is discharged from said catalyst stripper into a catalystregenerator; regenerating stripped catalyst in said catalyst regeneratorat catalyst regeneration conditions including contact with an oxygencontaining gas to produce regenerated catalyst which is recycled to saidcracking reactor; and transferring from said stripper to said closedcyclones at least a majority of said stripper vapor discharged up fromsaid fluidized bed in said stripper via a snorkel having a lower snorkelinlet above said dense phase of fluidized catalyst in said stripper, anupper snorkel outlet fluidly connected with said cyclone separators, anda vertical transfer conduit section fluidly isolated from said vesselcontaining said cyclone separators and physically attached to or withinat least one of said primary cyclone diplegs.

In an apparatus embodiment, the present invention provides an apparatusfor fluidized catalytic cracking of hydrocarbon feed comprising: areactor vessel, a riser reactor having a base section and an uppersection; an inlet in the base of the riser for the heavy feed; an inletin the base of the riser for a source of hot regenerated catalyticcracking catalyst; an outlet in the upper section of the riser fordischarging catalytically cracked products and spent catalyst into saidreactor vessel; a closed cyclone separation means within said vesselreceiving cracked products and spent catalyst from said riser forseparation of cracked products from spent catalyst; a spent catalyststripper means in a base portion of said reactor vessel beneath saidclosed cyclone having a spent catalyst inlet for catalyst from saidclosed cyclone, a stripping gas inlet in a lower portion thereof, astripper vapor outlet in an upper portion thereof and a strippedcatalyst outlet; a stripper vapor transfer conduit having an inlet inopen fluid communication with said vessel and an elevation intermediatesaid cyclonic separation means and said stripper vapor outlet and anoutlet sealingly affixed to said cyclone separation means for transferof stripper vapor to said cyclone separation means; a catalystregenerator having an inlet for an oxygen containing regeneration gas,an inlet for stripped catalyst from said stripper catalyst outlet, anoutlet for flue gas and an outlet for regenerated catalyst for recycleof regenerated catalyst connective with the base of the riser reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (prior art) is a schematic view of a conventional fluidizedcatalytic cracking unit.

FIG. 2 is a schematic view of one type of snorkel closed cyclone system.

FIG. 3 is a schematic view of a preferred snorkel closed cyclone system.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a simplified schematic view of an FCC unit of the prior art,similar to the Kellogg Ultra Orthoflow converter Model F shown as FIG.17 of Fluid Catalytic Cracking Report, in the Jan. 8, 1990 edition ofOil & Gas Journal and is identical to FIG. 1 of my U.S. Pat. No.5,346,610, which is incorporated by reference.

A heavy feed such as a gas oil or vacuum gas oil is added to riserreactor 6 via feed injection nozzles 2. The cracking reaction iscompleted in the riser reactor, which takes a 90□ turn at the top of thereactor at elbow 10. Spent catalyst and cracked products discharged fromthe riser reactor pass through riser cyclones 12 which efficientlyseparate most of the spent catalyst from cracked product. Crackedproduct is discharged into disengager 14, and eventually is removed viaupper cyclones 16 and conduit 18 to the fractionator.

Spent catalyst is discharged down from a dipleg of riser cyclones 12into catalyst stripper 8, where one, or preferably 2 or more, stages ofsteam stripping occur, with stripping steam admitted by steam inlet anddistributors 19 and 21 at lower and upper levels in the stripper. Thestripped hydrocarbons, and stripping steam, pass into disengager 14 andare removed with cracked products after passage through upper cyclones16.

Stripped catalyst is discharged down via spent catalyst standpipe 26into catalyst regenerator 24. The flow of catalyst is controlled withspent catalyst plug valve 36.

Catalyst is regenerated in regenerator 24 by contact with air, added viaair lines and an air grid distributor not shown. A catalyst cooler 28 isprovided so that heat may be removed from the regenerator, if desired.Regenerated catalyst is withdrawn from the regenerator via regeneratedcatalyst plug valve assembly 30 and discharged via lateral 32 into thebase of the riser reactor 6 to contact and crack fresh feed injected viainjectors 2, as previously discussed. Flue gas and some entrainedcatalyst are discharged into a dilute phase region in the upper portionof regenerator 24. Entrained catalyst is separated from flue gas inmultiple stages of cyclones 4, and discharged via outlets 8 into plenum20 for discharge to the flare via line 22.

The FIG. 1 design shows riser cyclones, but not closed cyclones. In aclosed cyclone design, the spent catalyst and cracked productsdischarged from the riser reactor pass through riser cyclones 12 withvapor passed via a pipe or conduit directly to upper cyclones 16.Provisions would be made to admit stripping gas somewhere to the closedcyclone system, preferably by having the vapor outlet from cyclone 12inserted loosely into the inlet to cyclone 16, so that stripping gassesmay pass through the annular opening created by the loose fit.

FIG. 1 shows the conventional approach to FCC riser reactors with closedcyclones. While markedly better than operating without closed cyclones,there is excessive thermal cracking, and diene production, in thereactor volume.

FIG. 2 shows a marked departure from conventional operation. FIG. 2 is asimplified view of part of the device, frequently multiple closedcyclones are used around the riser but only one is shown here.

An FCC riser reactor 200 discharged a mixture of spent catalyst andcracked products via line 210 into primary cyclone 215. A vapor phasewith greatly reduced solids content is removed via conduit 220 intosecondary cyclone 250. A solids rich phase is discharged from primarycyclone 215 via dipleg 225. This dipleg is sealed at the base byimmersion in the upper level of fluidized solids in the stripper, shownas region 290. The cyclone dipleg may also be sealed by a flapper valve.

Gas discharged up from region 290, the stripping steam and strippedhydrocarbons, passes into vapor space region 295 within the reactorvessel, not shown.

Normally stripper vapor has a long trip. In prior art units it has torise to an elevation where the stripper vapor can exit, either the inletto the secondary cyclones in an open cyclone system or to the openingfor stripping vapors in a closed cyclone system, an opening which istypically at an elevation near or above the reactor cyclones. In myprocess, the gas takes a short cut, in terms of residence time, byflowing through opening 285 in the base of snorkel 280 connective withline 220.

The flow of stripper vapor in this embodiment is up from region 290,typically 0.2 to 2 meters above the stripper catalyst level, into thesnorkel and out of the unit. The reactor vessel remains large, and somebeneficial use is made of the large void volume region 295, in providingsettling time so that much entrained catalyst exiting the stripper cansimply fall back into the stripper.

Catalyst entrainment, or the amount of catalyst per unit volume ofstripper vapor, will be higher in my design than in the prior art unitsbecause the dilute phase region near the stripper has more catalystpresent than the dilute phase region near the top of the reactor voidvolume. For this reason, I prefer to put snorkel 280 intermediate theprimary and secondary cyclones, so that any catalyst which is entrainedup the snorkel can be recovered by the secondary cyclone. If desired,snorkel 280 may discharge into a small cyclone (not shown), with thevapor phase from this cyclone passing into line 220 or line 260, oranywhere which permits rapid removal of vapor from the reactor.

The secondary cyclone 250 receives both the vapor phase discharged fromthe primary cyclone and the vapor/entrained catalyst recovered from thestripper via snorkel 280. The recovered solids are discharged via dipleg270 into the stripper, with the dipleg sealed by immersion in region290. The vapor phase is discharged via vapor outlet 260, which passesthrough an expansion bellows 265 to the transfer line to the FCC maincolumn.

FIG. 3 shows a preferred embodiment, which is easy to implement in manyrefinery installations. Only details around the primary cyclone 315 areshown. The primary cyclone receives cracked products and spent catalystdischarged via line 310 from the riser reactor, not shown. Most of thesolids are discharged down via dipleg 325, the outlet of which is sealedby immersion in the fluidized bed of catalyst 390 forming the top of thestripper. Stripper vapor enters snorkel 380 via inlet 385. What isunusual about this design is that the snorkel is within and axiallyaligned with the cyclone dipleg 325. Such a design greatly simplifiesfield installation of the snorkel, as the snorkel can be fabricated aspart of a new dipleg 325 and supported by it.

There may be a slight degradation in performance of the primary cyclonedue to the presence of the snorkel in central region, but the secondarycyclone efficiency will usually be high enough to permit installation ofthe snorkel.

It is beneficial to provide some means, not shown in the figures, todeal with the different rates of thermal expansion of various pieces ofequipment. I prefer to put an expansion joint downstream of the secondstage cyclone. Use of expansion joints is conventional in FCC units andper se forms no part of the present invention.

Having provided an overview of the cracking process and severalpreferred snorkel designs, more detailed information will be providedabout the different parts of the process and apparatus.

Cracking Catalyst

Conventional cracking catalysts may be used. Practically every FCC unitin the world uses zeolite Y cracking catalyst, and de-aluminized formsof this zeolite such as DEAL Y, USY, and even ultra-hydrophobic Y(UHP-Y) may be used, with or without rare earth stabilization. RE-USYbased cracking catalyst will be preferred by many refiners. The catalystmay also contain some shape selective zeolite such as ZSM-5, either asan integral part of the cracking catalyst or as a separate additive.

Catalyst, per se, forms no part of the present invention.

Cracking Conditions

The FCC unit may operate under conventional FCC conditions, including ariser top temperature in the range from about 100□ F. to about 1350□ F.,a Catalyst-to-Oil ratio from about 1:1 to about 20:1, and a contact timeof from about 0.1 to about 20 sec. The reactor conditions, per se, formno part of the present invention.

Cracking Feeds

Cracking feeds may be conventional, such as petroleum fractions havingan initial boiling point of at least 500□ F. (260□ C.), a 50% point atleast 750° F. (399□ C.), and an end point of at least 1100□ F. (593□C.). Such fractions include gas oils, vacuum gas oils, thermal oils,residual oils, cycle stocks, whole top crudes, tar sand oils, shaleoils, synthetic fuels, heavy hydrocarbon fractions derived from thedestructive dehydrogenation of coal, tar, pitches, asphalt,hydro-treated feedstocks derived from any of the foregoing, and thelike.

Closed Cyclone System

Any closed cyclone system can be used. Several have been developed, withthe primary difference being where stripping vapors enter the closedcyclone system to be mixed with cracked product vapors. One good closedcyclone design is disclosed in my U.S. Pat. No. 4,502,947 and itsdivisions and continuations including U.S. Pat. No. 5,039,397, which areincorporated by reference. My snorkel works well with this closedcyclone system, but can be used with other closed cyclone designs.

The generic characteristics of a closed cyclone system are:

1. cyclonic separation of cracked products from spent catalysts as thematerial exits the reactor within a vessel,

2. isolation of the recovered vapor from the interior atmosphere of thevessel, and usually

3. return of stripper vapor to the cyclone separation system.

It is possible to have a closed cyclone system attached to an up-flowriser reactor, a reactor with a horizontal discharge section, or even toa down-flow reactor. So long as the reactor discharges directly into acyclonic separation device condition #1 is satisfied.

In order for the cyclone system to be “closed” the vapor product fromthe primary cyclone must be isolated from the vessel atmosphere. Usuallythis is done by providing lines or conduits which physically isolate thevapor discharged from the primary (and secondary) cyclone(s) from thereactor vessel atmosphere as vapor passes from the cyclones via thetransfer line to the FCC main column.

Condition #3 calls for mixing the stripper vapor with the vapor chargedto the main column. I want to retain the simplicity and efficiency ofcombining stripper vapor and the bulk of the cracked vapor productsdischarged from the reactor. Although these must eventually be combinedon their way to the main column, there are several ways to do this.

Preferably this is done as shown in FIGS. 2 and 3, adding stripper vaporto the vapor line connecting the primary and secondary cyclones or intothe primary cyclone near the vapor outlet. This provides the bestcombination of pressure balance and simplicity.

It is also possible to send the stripper vapor to the inlet side of theprimary cyclone. This can be done with brute force by using a blower orsteam aspiration to provide motive force to get the stripper vapor intothe cyclone inlet, or by using a long dipleg to discharge solids fromthe primary cyclone. This allows the primary cyclone to run as anegative pressure cyclone so that gas will flow into it from both theriser and the opening for stripper vapor.

As previously discussed, it is preferred to send stripper vapor backbetween the primary and secondary cyclones.

It is possible to send stripper vapor to a point downstream of thesecondary reactor cyclone. This may increase catalyst entrainment intothe FCC main column.

A small cyclone(s) could be added to the snorkel line if it were desiredto remove entrained catalyst. This will be especially important if theentrained catalyst in the gas flowing through the snorkel is likely tocause difficulties in downstream processing units.

1. An apparatus for fluidized catalytic cracking of hydrocarbon feed comprising: (a.) a reactor vessel; (b.) an elongated reactor having an inlet section comprising an inlet for said hydrocarbon feed and an inlet for a source of hot regenerated catalytic cracking catalyst and an outlet section having an outlet discharging catalytically cracked products and spent catalyst into said reactor vessel; (c.) a closed cyclone separation means within said reactor vessel receiving cracked products and spent catalyst discharged from said elongated reactor and separating them into a catalyst rich phase which is discharged down into a catalyst stripper within said reactor vessel and a cracked product phase with a reduced content of catalyst which is discharged from said closed cyclone via a cyclone vapor outlet; (d.) a spent catalyst stripper means in a lower portion of said reactor vessel, receiving spent catalyst discharged from said closed cyclone, having a stripping gas inlet in a lower portion of said catalyst stripper, a stripper vapor outlet in an upper portion of said stripper means which is in open fluid communication with said reactor vessel and a stripped catalyst outlet in a lower portion of said stripper; (g.) a stripper vapor transfer conduit having an inlet in a lower portion thereof in open fluid communication with said reactor vessel at an elevation below said closed cyclone and above said catalyst stripper vapor outlet, and an outlet in an upper portion of said transfer conduit fluidly connected with said closed cyclone separator or a line fluidly connective with said cyclone vapor outlet; (h.) a catalyst regenerator means receiving stripped catalyst discharged from said catalyst stripper having an inlet for an oxygen-containing regeneration gas, an outlet for flue gas produced during catalyst regeneration and an outlet for regenerated catalyst connective with said inlet portion of said elongated reactor.
 2. The apparatus of claim 1 wherein said elongated reactor is a riser reactor.
 3. The apparatus of claim 1 wherein said closed cyclone separation means comprises at least one primary cyclone connected to said reactor, said primary cyclone discharging said catalyst rich phase down into said stripper via a primary cyclone catalyst standpipe and discharging vapor via a primary cyclone vapor conduit into at least one secondary cyclone, and said outlet of said stripper vapor transfer conduit is sealingly affixed to said primary cyclone vapor conduit.
 4. The apparatus of claim 1 wherein said cyclone separation means comprises at least one primary cyclone connected to said reactor discharging said catalyst rich phase down into said stripper via a catalyst standpipe and discharging vapor via a primary cyclone vapor conduit into at least one secondary cyclone, and wherein said stripper vapor transfer conduit has an outlet connective with said primary cyclone vapor conduit discharging into said secondary cyclone.
 5. The apparatus of claim 1 wherein said cyclone separation means comprises at least one primary cyclone connected to said reactor discharging solids down into said stripper via a catalyst standpipe and said stripper vapor transfer conduit is at least partially contained within said catalyst standpipe and said transfer conduit outlet is within said primary cyclone.
 6. An apparatus for fluidized catalytic cracking of hydrocarbon feed comprising: (a.) a reactor vessel; (b.) a riser reactor having inlets in a lower portion thereof for said hydrocarbon feed and for a source of hot regenerated catalytic cracking catalyst and an outlet in an upper portion thereof discharging catalytically cracked products and spent catalyst into an upper portion of said reactor vessel; (c.) a closed cyclone separation means within said reactor vessel receiving cracked products and spent catalyst discharged from said riser reactor and separating them into a catalyst rich phase which is discharged down a standpipe into a catalyst stripper within said reactor vessel and a cracked product phase with a reduced content of catalyst which is discharged from said closed cyclone via a cyclone vapor outlet; (d.) a spent catalyst stripper means in a lower portion of said reactor vessel, receiving spent catalyst discharged from said closed cyclone, having a stripping gas inlet in a lower portion of said catalyst stripper, a stripper vapor outlet in an upper portion of said stripper means which is in open fluid communication with said reactor vessel and a stripped catalyst outlet in a lower portion of said stripper; (g.) a stripper vapor transfer conduit having an inlet in a lower portion thereof in open fluid communication with said reactor vessel at an elevation below said closed cyclone and above said catalyst stripper vapor outlet, and an outlet in an upper portion of said transfer conduit fluidly connected with said closed cyclone separator or a line fluidly connective with said cyclone vapor outlet; (h.) a catalyst regenerator means receiving stripped catalyst discharged from said catalyst stripper having an inlet for an oxygen-containing regeneration gas, an outlet for flue gas produced during catalyst regeneration and an outlet for regenerated catalyst connective with said inlet portion of said riser reactor.
 7. The apparatus of claim 6 wherein said standpipe discharges down into said catalyst stripper and said standpipe is sealed by immersion in said stripper.
 8. The apparatus of claim 6 wherein said standpipe terminates at an elevation above said catalyst stripper and said standpipe is sealed with a flapper valve.
 9. The apparatus of claim 6 wherein said stripper vapor transfer conduit is attached to said standpipe.
 10. The apparatus of claim 6 wherein said stripper vapor transfer conduit is within and axially aligned with said standpipe.
 11. The apparatus of claim 6 wherein multiple primary cyclones, in parallel, are used to separate spent catalyst and cracked product discharged from said riser reactor, and multiple stripper standpipes discharge a spent catalyst phase down into said catalyst stripper. 